Summary

For video drivers that support kernel mode-setting (KMS), X can be set up to run as a non-root user.

Background

Historically, X has been responsible for setting up the graphics modes (resolutions, refresh rates, etc.) X did this by talking to the hardware directly, which it could only do if it ran with root privileges. The reason X was tasked with doing this work was to keep graphics as platform-agnostic as possible, so the same graphics code could be used for BSD and other *nix flavors.

Today, the feeling is that this mode-setting logic should be moved into the kernel. Several video drivers now have this 'kernel mode-setting' (KMS) ability implemented. In lucid, -intel, -nouveau, and -radeon all use KMS. But other drivers, including those for uncommon hardware and the proprietary -fglrx and -nvidia, do not support KMS at all.

Discussion

While the kernel and Xorg are set up to run X without root permissions, device permissions and other plumbing work needs done. Some decisions need made such as where X should store its log files, and whether it should run as the user, or if a special user or group should be created for X.

For single user devices (where there is no login screen or account management), the simplest approach would be to use that user and store files in its user space.

For multi-user situations (such as a traditional desktop), either X could run as a service and flip its ownership to the logged in user via udev rules, or else the login session should run as one X session and allow for instantiating secondary X sessions as the user. This latter approach is most secure, but may be tougher to ensure a smooth transition.

What user should X be run as?

> $ sudo chown $USER /dev/tty8 > $ sudo chmod o+rw /dev/input/*

This is the main thing to be solved, and I think it's doable with only minor additions.

Right now, root launches the gdm greeter, running as "gdm", which launches the setuid X. Once the user authenticates, the cookie is transfered to the new user, and things continue. However, this means the same X server is running for both users (a design issue that I believe is a risk, actually).

So, the question here for rootless X needs to be: "what user should X run as?"

If it should match the logged-in user, then X needs to be restarted once the gdm-greeter authenticates the new user. This is rather different from the current design. To handle the permissions, the gdm start-up just needs to add file ACL perms for the "gdm" user to /dev/tty7 and /dev/input/* in the same way that HAL used to add ACLs for things like FUSE, scanners, etc, and then once the new user auths, transfer the permissions instead of the cookie, and let the user start X.

setfactl -m user:$USER:rw /dev/tty7 $(find -type c /dev/input)

Additionally, udev would need to be modified to add these ACLs for as long as the user is considered "on the local console" so that newly attached input devices are available to the user.

If, on the other hand, we want X to effectively remain a "service", running as a different user (and non-root), then we just need to add udev rules to flip the ownership to that user, and everything stays the same, except that there is a new system user who's entire purpose is to run X.

The latter option is by far the simplest to implement but still creates a situation where finding flaws in the X server leads to potential exploitation of other users that log in to X. As such, I would prefer we find a way to do the former, but the latter is certainly a good first-step for a working proof-of-concept.

The work for the former is also, I think, possible to be done in parallel with a regular setuid X. My only question is how input devices are currently shared between simultaneous X sessions. And perhaps it creates a situation where users can snoop on the input from other user sessions? And what kind of expense is there in restarting the X process if we launch it once for the gdm greeter, and then again for the authenticated user?

-Kees

Permissions

Bryce Harrington [2010-02-06 0:08 -0800]: > As I've mentioned previously, *most* of the work is getting permissions > sorted out on device files. This needs a foundations type person (like > Steve Langasek or Martin Pitt) to figure out. Alberto and I succeeded > in hacking around this today, but the hack we came up with, while it > works, I think it may actually make the system *less* secure than if we > ran X as root. It would be a good idea to have someone from foundations > put some thought into how we can do this in a proper manner.

As far as I remember, there were two major problems:

(1) input device access

We really shouldn't allow access to /dev/input/* to normal users, since that would make it way too easy to install key loggers and so on. Solutions:

Run X.org not as the human user, but as a system user in an "input"

group, and install udev rules to make the devices root:input 664.

(Slight variation) Make X.org setgid "input", same udev rules.

(2) backlight access

Since these aren't device files, but kernel bits in /sys, we cannot control these with userspace magic like udev/ConsoleKit/etc. Solutions:

Change kernel drive to expose a real device node for this for

controlling, instead of writing to /sys, and use udev's automatic ACLs to grant access to current foreground user. IANAKD, so I have no idea how much effort this is.

Split out a kind of "ubacklight" service which runs as root,

writes /sys, and uses the usual ConsoleKit/PolicyKit policy of allowing access to the current foreground console. X.org would then talk to it through D-Bus. Quite some overhead, though.

Martin

Upgrade Considerations

There is another issue here, which is that if the -intel driver in Lucid doesn't support UMS, the kernel and X have to be upgraded in lockstep for LTS->LTS upgrades. A power event mid-upgrade (where *either* of the kernel or the X packages upgraded without the other) will leave the system unable to bring X up, which means desktop users will have a very hard time completing the upgrade.

If we keep the support for UMS around through Lucid (and drop it in Lucid+1), then we have a reliable upgrade path because the new X driver works with both the old and new kernels, so the user can recover from a power event by booting the previous kernel.

I think the right way round is for Lucid to have UMS support re-enabled, and to have a KMS-only package in PPA. Having UMS support in a PPA is clearly not going to help users in the middle of an upgrade from hardy.

=== Revoke Syscall ==

Kees's summary of the plumbing requirements for X-as-service is accurate (udev rules to grant access to the input and vt devices to just the service user).

To safely permit the individual user accounts to run an X server would require implementation of a "revoke" syscall in the kernel, to ensure that each server has exclusive access to the input devices when it's in the foreground; and a root daemon (perhaps consolekit) to arbitrate this access on behalf of the servers. I strongly counsel against attempting anything of this sort on a Lucid time frame. The lack of a revoke syscall in Linux has been a known limitation for over a decade, it's not something that's going to be fixed right (securely) in three months - rushing that implementation is only going to *decrease* security.